Ethylene-Alpha olefin copolymers and polymerization processes for making the same

ABSTRACT

A process for the production of an ethylene alpha-olefin copolymer is disclosed. The process includes polymerizing ethylene and at least one alpha-olefin by contacting the ethylene and the at least one alpha-olefin with a metallocene catalyst in at least one gas phase reactor at a reactor pressure of from 0.7 to 70 bar and a reactor temperature of from 20° C to 150° C to form an ethylene alpha-olefin copolymer. The resulting ethylene alpha-olefin copolymer may have a density of 0.927 g/cc or greater and environmental stress crack resistance (ESCR) of 500 hr or more when measured according to ASTM 1693/B in 10% Igepal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Application Nos. 60/816,840,filed Jun. 27, 2006, and 60/858,825, filed Nov. 14, 2006, thedisclosures of which are herein incorporated in their entireties.

FIELD OF THE INVENTION

The invention relates generally to ethylene alpha olefin copolymershaving densities of about 0.927 g/cm³ or higher and processes for makingthe same.

BACKGROUND OF THE INVENTION

The composition distribution of an ethylene alpha-olefin copolymerrefers to the distribution of comonomer (short chain branches) among themolecules that comprise the polyethylene polymer. When the amount ofshort chain branches varies among the polyethylene molecules, the resinis said to have a “broad” composition distribution. When the amount ofcomonomer per 1000 carbons is similar among the polyethylene moleculesof different chain lengths, the composition distribution is said to be“narrow”.

The composition distribution is known to influence the properties ofcopolymers, for example, extractables content, environmental stresscrack resistance, heat sealing, and tear strength. The compositiondistribution of a polyolefin may be readily measured by methods known inthe art, for example, Temperature Raising Elution Fractionation (TREF)or Crystallization Analysis Fractionation (CRYSTAF).

Ethylene alpha-olefin copolymers are typically produced in a lowpressure reactor, utilizing, for example, solution, slurry, or gas phasepolymerization processes. Polymerization takes place in the presence ofcatalyst systems such as those employing, for example, a Ziegler-Nattacatalyst, a chromium based catalyst, a metallocene catalyst, orcombinations thereof.

It is generally known in the art that a polyolefin's compositiondistribution is largely dictated by the type of catalyst used andtypically invariable for a given catalyst system. Ziegler-Nattacatalysts and chromium based catalysts produce resins with broadcomposition distributions (BCD), whereas metallocene catalysts normallyproduce resins with narrow composition distributions (NCD).

Resins having a Broad Orthogonal Composition Distribution (BOCD) inwhich the comonomer is incorporated predominantly in the high molecularweight chains can lead to improved physical properties, for exampletoughness properties and Environmental Stress Crack Resistance (ESCR).

Because of the improved physical properties of resins with orthogonalcomposition distributions needed for commercially desirable products,there exists a need for medium and high density polyethylenes having anorthogonal composition distribution.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to a process for theproduction of an ethylene alpha-olefin copolymer. The process mayinclude polymerizing ethylene and at least one alpha-olefin bycontacting the ethylene and the at least one alpha-olefin with ametallocene catalyst in at least one gas phase reactor at a reactorpressure of between 0.7 and 70 bar and a reactor temperature of between20° C. and 150° C. to form an ethylene alpha-olefin copolymer. Theresulting ethylene alpha-olefin copolymer may have a density of 0.927g/cc or higher, a melt index (I₂) of between 0.1 and 100 dg/min, a meltindex ratio of I₂₁/I₂ between 15 and 40, an ESCR value of 500 hr orgreater when measured according to ASTM 1693/B in 10% Igepal, and anorthogonal composition distribution evidenced by a M₆₀/M₉₀ value ofgreater than 1, wherein M₆₀ is the molecular weight of the polymerfraction that elutes at 60° C. and M₆₀ is the molecular weight of thepolymer fraction that elutes at 90° C. in a TREF-LS experiment.

In another aspect, the present invention relates to a process for theproduction of an ethylene alpha-olefin copolymer having a broadcomposition distribution. The process may include polymerizing ethyleneand at least one alpha-olefin by contacting the ethylene and the atleast one alpha-olefin with a metallocene catalyst in at least one gasphase reactor at a reactor pressure of between 0.7 and 70 bar and areactor temperature of between 20° C. and 150° C. to form an ethylenealpha-olefin copolymer. The resulting ethylene alpha-olefin copolymermay have a density of 0.927 g/cc or higher, a melt index (I₂) of between0.1 and 100 dg/min, a melt index ratio of I₂₁/I₂ between 15 and 40, anda broad composition distribution evidenced by a T₇₅-T₂₅ value of greaterthan 15 wherein T₂₅ is the temperature at which 25% of the elutedpolymer is obtained and T₇₅ is the temperature at which 75% of theeluted polymer is obtained in a TREF experiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents the TREF-LS results obtained for Sample 1.

FIG. 2 presents the TREF-LS results obtained for Sample 2.

FIG. 3 presents the TREF-LS results obtained for Sample 3.

FIG. 4 presents the TREF-LS results obtained for Sample 4.

FIG. 5 presents the TREF-LS results obtained for Sample 5.

FIG. 6 presents the TREF-LS results obtained for the comparative sample,Sample 6.

FIG. 7 is a schematic representation of the variations of compositiondistribution.

DETAILED DESCRIPTION

Before the present compounds, components, compositions, and/or methodsare disclosed and described, it is to be understood that unlessotherwise indicated this invention is not limited to specific compounds,components, compositions, reactants, reaction conditions, ligands,metallocene structures, or the like, as such may vary, unless otherwisespecified. It is also to be understood that the terminology used hereinis for the purpose of describing particular embodiments only and is notintended to be limiting.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless otherwise specified. Thus, for example, reference to “aleaving group” as in a moiety “substituted with a leaving group”includes more than one leaving group, such that the moiety may besubstituted with two or more such groups. Similarly, reference to “ahalogen atom” as in a moiety “substituted with a halogen atom” includesmore than one halogen atom, such that the moiety may be substituted withtwo or more halogen atoms, reference to “a substituent” includes one ormore substituents, reference to “a ligand” includes one or more ligands,and the like.

Embodiments of this invention relate to the production of ethylenealpha-olefin copolymers as well as improvements in the properties of theresulting ethylene alpha-olefin copolymer.

In other embodiments, the invention relates to metallocene catalysts andpolymerization processes for producing a metallocene polyethylene havinga density of 0.927 g/cc or higher with an orthogonal compositiondistribution.

In other aspects, embodiments disclosed herein relate to an ethylenealpha-olefin copolymer having a density of 0.927 g/cc or higher with abroad composition distribution.

Composition Distribution

The composition distribution of an ethylene alpha-olefin copolymerrefers to the distribution of comonomer (short chain branches) among themolecules that comprise the polyethylene polymer. Ziegler-Nattacatalysts and chromium based catalysts produce resins with BroadComposition Distributions (BCD). These Ziegler-Natta and chromium basedBCD resins are further characterized by a “conventional comonomerincorporation”. What is meant by “conventional comonomer incorporation”is that the comonomer is incorporated predominantly in the low molecularweight chains. FIG. 7 is provided to further illustrate the conceptspresented in this section.

Certain metallocene catalysts are capable of producing resins withnarrow composition distributions (NCD), in which the comonomer contentis about uniform among the polymer chains of different molecularweights.

BOCD refers to a Broad Orthogonal Composition Distribution in which thecomonomer is incorporated predominantly in the high molecular weightchains. The distribution of the short chain branches can be measured,for example, using Temperature Raising Elution Fractionation (TREF) inconnection with a Light Scattering (LS) detector to determine the weightaverage molecular weight of the molecules eluted from the TREF column ata given temperature. The combination of TREF and LS (TREF-LS) yieldsinformation about the breadth of the composition distribution andwhether the comonomer content increases, decreases, or is uniform acrossthe chains of different molecular weights.

Certain advantages of a broad orthogonal composition distribution (BOCD)for improved physical properties and low extractables content aredisclosed in, for example, U.S. Pat. No. 5,382,630.

The TREF-LS data reported herein were measured using an analytical sizeTREF instrument (Polymerchar, Spain), with a column of the followingdimension: inner diameter (ID) 7.8 mm and outer diameter (OD) 9.53 mmand a column length of 150 mm. The column was filled with steel beads.0.5 mL of a 6.4% (w/v) polymer solution in orthodichlorobenzene (ODCB)containing 6 g BHT/4 L were charged onto a the column and cooled from140° C. to 25° C. at a constant cooling rate of 1.0° C./min.Subsequently, ODCB was pumped through the column at a flow rate of 1.0ml/min, and the column temperature was increased at a constant heatingrate of 2° C./min to elute the polymer. The polymer concentration in theeluted liquid was detected by means of measuring the absorption at awavenumber of 2857 cm⁻¹ using an infrared detector. The concentration ofthe ethylene-α-olefin copolymer in the eluted liquid was calculated fromthe absorption and plotted as a function of temperature. The molecularweight of the ethylene-α-olefin copolymer in the eluted liquid wasmeasured by light scattering using a Minidawn Tristar light scatteringdetector (Wyatt, Calif. USA). The molecular weight was also plotted as afunction of temperature.

The breadth of the composition distribution is characterized by theT₇₅-T₂₅ value, wherein T₂₅ is the temperature at which 25% of the elutedpolymer is obtained and T₇₅ is the temperature at which 75% of theeluted polymer is obtained in a TREF experiment as described herein. Thecomposition distribution is further characterized by the F₈₀ value,which is the fraction of polymer molecules that elute below 80° C. in aTREF-LS experiment as described herein. A higher F₈₀ value indicates ahigher fraction of comonomer in the polymer molecule. An orthogonalcomposition distribution is defined by a M₆₀/M₆₀ value that is greaterthan 1, wherein M₆₀ is the molecular weight of the polymer fraction thatelutes at 60° C. in a TREF-LS experiment and M₉₀ is the molecular weightof the polymer fraction that elutes at 90° C. in a TREF-LS experiment asdescribed herein.

Metallocene Catalyst Compounds

The metallocene catalyst compounds as described herein include “halfsandwich” and “full sandwich” compounds having one or more Cp ligands(cyclopentadienyl and ligands isolobal to cyclopentadienyl) bound to atleast one Group 3 to Group 12 metal atom, and one or more leaving groupsbound to the at least one metal atom. Hereinafter, these compounds willbe referred to as “metallocenes” or “metallocene catalyst components”.The metallocene catalyst component may be supported on a supportmaterial, as described further below, and may be supported with orwithout another catalyst component. In one embodiment, the one or moremetallocene catalyst components of the invention are represented by theformula (I):Cp^(A)Cp^(B)MX_(n)   (I)

wherein M is a metal atom selected from the group consisting of Groups 3through 12 atoms and lanthanide Group atoms in one embodiment. In otherembodiments, M may be selected from Ti, Zr, Hf atoms. In yet otherembodiments, M is hafnium (Hf). Each leaving group X is chemicallybonded to M; each Cp group is chemically bonded to M; and n is 0 or aninteger from 1 to 4, and either 1 or 2 in a particular embodiment.

The Cp ligands are one or more rings or ring systems, at least a portionof which includes π-bonded systems, such as cycloalkadienyl ligands andheterocyclic analogues. The Cp ligands are distinct from the leavinggroups bound to the catalyst compound in that they are not highlysusceptible to substitution or abstraction reactions. The ligandsrepresented by Cp^(A) and Cp^(B) in formula (I) may be the same ordifferent cyclopentadienyl ligands or ligands isolobal tocyclopentadienyl, either or both of which may contain heteroatoms andeither or both of which may be substituted by at least one R group.Non-limiting examples of substituent R groups include groups selectedfrom hydrogen radicals, alkyls, alkenyls, alkynyls, cycloalkyls, aryls,acyls, aroyls, alkoxys, aryloxys, alkylthiols, dialkylamines,alkylamidos, alkoxycarbonyls, aryloxycarbonyls, carbomoyls, alkyl- anddialkyl-carbamoyls, acyloxys, acylaminos, aroylaminos, and combinationsthereof. In one embodiment, Cp^(A) and Cp^(B) are independently selectedfrom the group consisting of cyclopentadienyl, indenyl,tetrahydroindenyl, fluorenyl, and substituted derivatives of each. (Asused herein, the term “substituted” means that the group following thatterm possesses at least one moiety in place of one or more hydrogens inany position, which moieties are selected from such groups as halogenradicals (e.g., Cl, F, Br), hydroxyl groups, carbonyl groups, carboxylgroups, amine groups, phosphine groups, alkoxy groups, phenyl groups,naphthyl groups, C₁ to C₁₀ alkyl groups, C₂ to C₁₀ alkenyl groups, andcombinations thereof. Examples of substituted alkyls and aryls include,but are not limited to, acyl radicals, alkylamino radicals, alkoxyradicals, aryloxy radicals, alkylthio radicals, dialkylamino radicals,alkoxycarbonyl radicals, aryloxycarbonyl radicals, carbomoyl radicals,alkyl- and dialkyl-carbamoyl radicals, acyloxy radicals, acylaminoradicals, arylamino radicals, and combinations thereof.).

In one embodiment, each leaving group X in the formula (I) above may beindependently selected from the group consisting of halogen ions,hydrides, C₁₋₁₂ alkyls, C₂₋₁₂ alkenyls, C₆₋₁₂ aryls, C₇₋₂₀ alkylaryls,C₁₋₁₂ alkoxys, C₆₋₁₆ aryloxys C₇₋₁₈ alkylaryloxys, C₁₋₁₂ fluoroalkyls,C₆₋₁₂ fluoroaryls, and C₁₋₁₂ heteroatom-containing hydrocarbons, andsubstituted derivatives thereof. As used herein, the phrase “leavinggroup” refers to one or more chemical moieties bound to the metal centerof the catalyst component, which can be abstracted from the catalystcomponent by an activator, thus producing a species active towardsolefin polymerization or oligomerization. The activator is describedfurther below.

The structure of the metallocene catalyst component may take on manyforms, such as those disclosed in, for example, U.S. Pat. No. 5,026,798,U.S. Pat. No. 5,703,187, and U.S. Pat. No. 5,747,406, including a dimeror oligomeric structure, such as disclosed in, for example, U.S. Pat.No. 5,026,798 and U.S. Pat. No. 6,069,213. Others include thosecatalysts describe in published U.S. Pat. App. Nos. US2005/0124487A1,US2005/0164875A1, and US2005/0148744. In other embodiments, themetallocene may be formed with a hafnium metal atom, such as isdescribed in U.S. Pat. No. 6,242,545.

In certain embodiments, the metallocene catalysts components describedabove may include their structural or optical or enantiomeric isomers(racemic mixture), and, in one embodiment, may be a pure enantiomer. Asused herein, a single, bridged, asymmetrically substituted metallocenecatalyst component having a racemic and/or meso isomer does not, itself,constitute at least two different bridged, metallocene catalystcomponents.

In one embodiment, the metallocene catalyst contains hafnium as themetal atom. In other embodiments, at least one of the ligands (pi-bondedmoieties) contains a cyclopentadienyl group. In other embodiments, themetallocene contains a chloride leaving group. In yet other embodiments,the metallocene contains a fluoride leaving group. In yet otherembodiments, the metallocene contains a methyl leaving group.

In some embodiments, the metallocene catalyst may be abis(n-propylcyclopentadienyl)hafnium X_(n),bis(n-butylcyclopentadienyl)hafnium X_(n),bis(n-pentylcyclopentadienyl)hafnium X_(n), (n-propylcyclopentadienyl)(n-butylcyclopentadienyl)hafnium X_(n),bis[(2-trimethylsilylethyl)cyclopentadienyl]hafnium X_(n),bis(trimethylsilyl cyclopentadienyl)hafnium X_(n),dimethylsilylbis(n-propylcyclopentadienyl)hafnium X_(n),dimethylsilylbis(n-butylcyclopentadienyl)hafnium X_(n),bis(1-n-propyl-2-methylcyclopentadienyl)hafnium X_(n),(n-propylcyclopentadienyl)(1-n-propyl-3-n-butylcyclopentadienyl)hafniumX_(n), or combinations thereof, where X_(n) is as described above.

In other embodiments, the metallocene catalyst may be abis(n-propylcyclopentadienyl)hafnium dichloride, abis(n-propylcyclopentadienyl)hafnium difluoride, or a dimethylbis(n-propylcyclopentadienyl)hafnium.

Activator and Activation Methods for the Metallocene Catalyst Compounds

The term “activator” is defined to be any compound or component whichcan activate a transition metal metallocene-type catalyst compound asdescribed above, for example, a Lewis acid or a non-coordinating ionicactivator or ionizing activator or any other compound that can convert aneutral metallocene catalyst component to a metallocene cation. It iswithin the scope of this invention to use alumoxane or modifiedalumoxane as an activator, and/or to also use ionizing activators,neutral or ionic, such astri(n-butyl)ammoniumtetrakis(pentafluorophenyl)boron or atrisperfluorophenyl boron metalloid precursor which ionize the neutralmetallocene compound. A preferred activator used with the catalystcompositions of the present invention is methylaluminoxane (“MAO”). TheMAO activator may be associated with or bound to a support, either inassociation with the catalyst component (e.g., metallocene) or separatefrom the catalyst component, such as described by Gregory G. Hlatky,Heterogeneous Single-Site Catalysts for Olefin Polymerization, 100(4)CHEMICAL REVIEWS 1347-1374 (2000).

There are a variety of methods for preparing alumoxane and modifiedalumoxanes, non-limiting examples of which are described in U.S. Pat.Nos. 4,665,208, 4,952,540, 5,091,352, 5,206,199, 5,204,419, 4,874,734,4,924,018, 4,908,463, 4,968,827, 5,308,815, 5,329,032, 5,248,801,5,235,081, 5,157,137, 5,103,031, 5,391,793, 5,391,529, 5,693,838 andEuropean publications EP-A-0 561 476, EP-B1-0 279 586 and EP-A-0594-218, and PCT publication WO 94/10180.

Ionizing compounds may contain an active proton, or some other cationassociated with but not coordinated or only loosely coordinated to theremaining ion of the ionizing compound. Such compounds and the like aredescribed in European publications EP-A-0 570 982, EP-A-0 520 732,EP-A-0 495 375, EP-A-0 426.637, EP-A-500 944, EP-A-0 277 003 and EP-A-0277 004, and U.S. Pat. Nos. 5,153,157, 5,198,401, 5,066,741, 5,206,197,5,241,025, 5,387,568, 5,384,299 and 5,502,124. Combinations ofactivators are also contemplated by the invention, for example,alumoxanes and ionizing activators in combinations, see for example, PCTpublications WO 94/07928 and WO 95/14044 and U.S. Pat. Nos. 5,153,157and 5,453,410.

Method for Supporting

A support may also be present as part of the catalyst system of thepresent invention. Supports, methods of supporting, modifying, andactivating supports for single-site catalyst such as metallocenes arediscussed in, for example, 1 METALLOCENE-BASED POLYOLEFINS 173-218 (J.Scheirs & W. Kaminsky eds., John Wiley & Sons, Ltd. 2000). The terms“support” or “carrier,” as used herein, are used interchangeably andrefer to any support material, including inorganic or organic supportmaterials. In one embodiment, the support material may be a poroussupport material. Non-limiting examples of support materials includeinorganic oxides and inorganic chlorides, and in particular suchmaterials as talc, clay, silica, alumina, magnesia, zirconia, ironoxides, boria, calcium oxide, zinc oxide, barium oxide, thoria, aluminumphosphate gel, and polymers such as polyvinylchloride and substitutedpolystyrene, functionalized or crosslinked organic supports such aspolystyrene divinyl benzene polyolefins or polymeric compounds, andmixtures thereof, and graphite, in any of its various forms.

Desirable carriers are inorganic oxides that include Group 2, 3, 4, 5,13 and 14 oxides and chlorides. Support materials include silica,alumina, silica-alumina, magnesium chloride, graphite, and mixturesthereof in one embodiment. Other useful supports include magnesia,titania, zirconia, montmorillonite (as described in EP0511665B1),phyllosilicate, and the like. In other embodiments, combinations of thesupport materials may be used, including, but not limited to,combinations such as silica-chromium, silica-alumina, silica-titania,and the like. Additional support materials may include those porousacrylic polymers described in EP0767184B 1.

The catalyst system of the invention can be made and used in a varietyof different ways. In one embodiment, the catalyst is unsupported,preferably in liquid form such as described in U.S. Pat. Nos. 5,317,036and 5,693,727 and European publication EP-A-0593083. In the preferredembodiment, the catalyst system of the invention is supported. Examplesof supporting the catalyst system used in the invention are described inU.S. Pat. Nos. 4,701,432, 4,808,561, 4,912,075, 4,925,821, 4,937,217,5,008,228, 5,238,892, 5,240,894, 5,332,706, 5,346,925, 5,422,325,5,466,649, 5,466,766, 5,468,702, 5,529,965, 5,554,704, 5,629,253,5,639,835, 5,625,015, 5,643,847, 5,665,665, 5,468,702, 6,090,740 and PCTpublications WO 95/32995, WO 95/14044, WO 96/06187, and WO 97/02297.

In another embodiment, the catalyst system of the invention contains apolymer bound ligand as described in U.S. Pat. No. 5,473,202. In oneembodiment the catalyst system of the invention is spray dried asdescribed in U.S. Pat. No. 5,648,310. In an embodiment the support ofthe invention is functionalized as described in European publicationEP-A-0802203 or at least one substituent or leaving group is selected asdescribed in U.S. Pat. No. 5,688,880.

In another embodiment of the invention, the supported catalyst system ofthe invention includes an antistatic agent or surface modifier, forexample, those described in U.S. Pat. No. 5,283,278 and PCT publicationWO 96/11960.

A preferred method for producing the catalyst of the invention can befound in WO 96/00245 and WO 96/00243.

Polymerization Process

The polymerization process of the present invention may be carried outusing any suitable process, such as, for example, solution, slurry, highpressure, and gas phase. A particularly desirable method for producingpolyolefin polymers according to the present invention is a gas phasepolymerization process preferably utilizing a fluidized bed reactor.This type reactor, and means for operating the reactor, are describedin, for example, U.S. Pat. Nos. 3,709,853; 4,003,712; 4,011,382;4,302,566; 4,543,399; 4,882,400; 5,352,749; 5,541,270; EP-A-0 802 202and Belgian Patent No. 839,380. These patents disclose gas phasepolymerization processes wherein the polymerization medium is eithermechanically agitated or fluidized by the continuous flow of the gaseousmonomer and diluent.

Other gas phase processes contemplated by the process of the inventioninclude series or multistage polymerization processes. Also gas phaseprocesses contemplated by the invention include those described in U.S.Pat. Nos. 5,627,242, 5,665,818 and 5,677,375, and European publicationsEP-A-0 794 200 EP-B1-0 649 992, EP-A-0 802 202 and EP-B-634 421.

In general, the polymerization process may be a continuous gas phaseprocess, such as a fluid bed process. A fluid bed reactor for use in theprocess of the present invention typically has a reaction zone and aso-called velocity reduction zone. The reaction zone includes a bed ofgrowing polymer particles, formed polymer particles and a minor amountof catalyst particles fluidized by the continuous flow of the gaseousmonomer and diluent to remove heat of polymerization through thereaction zone. The gas leaving the reaction zone is passed to thevelocity reduction zone where entrained particles are allowed to settleback to the particle bed. Finer entrained particles and dust may beremoved in a cyclone and/or fine filter. The gas is passed through aheat exchanger wherein the heat of polymerization is removed, compressedin a compressor and then returned to the reaction zone. Optionally, someof the recirculated gases may be cooled and compressed to form liquidsthat increase the heat removal capacity of the circulating gas streamwhen readmitted to the reaction zone. A suitable rate of gas flow may bereadily determined by simple experiment. Makeup of gaseous monomer tothe circulating gas stream is at a rate equal to the rate at whichparticulate polymer product and monomer associated therewith iswithdrawn from the reactor, and the composition of the gas passingthrough the reactor is adjusted to maintain an essentially steady stategaseous composition within the reaction zone.

The process of the present invention is suitable for the production ofhomopolymers of olefins, including ethylene, and/or copolymers,terpolymers, and the like, of olefins, including polymers comprisingethylene and at least one or more other olefins. The olefins may bealpha-olefins, such as propylene, butane, hexene, or mixtures thereof.The olefins, for example, may contain from 2 to 16 carbon atoms in oneembodiment; ethylene and a comonomer comprising from 3 to 12 carbonatoms in another embodiment; ethylene and a comonomer comprising from 4to 10 carbon atoms in another embodiment; and ethylene and a comonomercomprising from 4 to 8 carbon atoms in another embodiment.

Other monomers useful in the process described herein includeethylenically unsaturated monomers, diolefins having 4 to 18 carbonatoms, conjugated or non-conjugated dienes, polyenes, vinyl monomers andcyclic olefins. Non-limiting monomers useful in the invention mayinclude norbornene, norbornadiene, isobutylene, isoprene,vinylbenzocyclobutane, styrenes, alkyl substituted styrene, ethylidenenorbornene, dicyclopentadiene and cyclopentene. In another embodiment ofthe process described herein, ethylene or propylene may be polymerizedwith at least two different comonomers, optionally one of which may be adiene, to form a terpolymer.

In one embodiment, the content of the alpha-olefin incorporated into thecopolymer may be less than 30 mol % in total; less than 20 mol % inother embodiments and less than 10 mol % in yet other embodiments. Theterm “polyethylene” when used herein is used. generically to refer toany or all of the polymers comprising ethylene described above.

Hydrogen gas is often used in olefin polymerization to control the finalproperties of the polyolefin. Using the catalyst system of the presentinvention, it is known that increasing the concentration (partialpressure) of hydrogen may increase the melt index ratio and/or meltindex (MI) of the polyolefin generated. The MFI or MI can thus beinfluenced by the hydrogen concentration. The amount of hydrogen in thepolymerization can be expressed as a mole ratio relative to the totalpolymerizable monomer, for example, ethylene, or a blend of ethylene andhexene or propylene. The amount of hydrogen used in the polymerizationprocesses of the present invention is an amount necessary to achieve thedesired MFI or MI of the final polyolefin resin.

Further, in certain embodiments, the polymerization process may includetwo or more reactors. Such commercial polymerization systems aredescribed in, for example, 2 METALLOCENE-BASED POLYOLEFINS 366-378 (JohnScheirs & W. Kaminsky, eds. John Wiley & Sons, Ltd. 2000); U.S. Pat. No.5,665,818, U.S. Pat. No. 5,677,375, and EP-A-0 794 200.

In one embodiment, the one or more reactors in a gas phase or fluidizedbed polymerization process may have a pressure ranging from about 0.7 toabout 70 bar (about 10 to 1000 psia); and in another embodiment, apressure ranging from about 14 to about 42 bar (about 200 to about 600psia). In one embodiment, the one or more reactors may have atemperature ranging from about 10° C. to about 150° C.; and in anotherembodiment from about 40° C. to about 125° C. In one embodiment, thereactor temperature may be operated at the highest feasible temperaturetaking into account the sintering temperature of the polymer within thereactor. In one embodiment, the superficial gas velocity in the one ormore reactors may range from about 0.2 to 1.1 meters/second (0.7 to 3.5feet/second); and from about 0.3 to 0.8 meters/second (1.0 to 2.7feet/second) in another embodiment.

In another embodiment of the invention, the polymerization process is acontinuous gas phase process that includes the steps of: (a) introducingethylene and at least one other alpha olefin monomer(s) into thereactor; (b) introducing the supported catalyst system; (c) withdrawinga recycle stream from the reactor; (d) cooling the recycle stream; (e)introducing into the reactor additional monomer(s) to replace themonomer(s) polymerized; (f) reintroducing the recycle stream or aportion thereof into the reactor; and (g) withdrawing a polymer productfrom the reactor.

In embodiments of the invention, one or more olefins, C₂ to C₃₀ olefinsor alpha-olefins, including ethylene or propylene or combinationsthereof, may be prepolymerized in the presence of the metallocenecatalyst systems described above prior to the main polymerization. Theprepolymerization may be carried out batch-wise or continuously in gas,solution or slurry phase, including at elevated pressures. Theprepolymerization can take place with any olefin monomer or combinationand/or in the presence of any molecular weight controlling agent such ashydrogen. For examples of prepolymerization procedures, see U.S. Pat.Nos. 4,748,221, 4,789,359, 4,923,833, 4,921,825, 5,283,278 and 5,705,578and European publication EP-B-0279 863 and PCT Publication WO 97/44371.

The present invention is not limited to any specific type of fluidizedor gas phase polymerization reaction and can be carried out in a singlereactor or multiple reactors such as two or more reactors in series. Inembodiments, the present invention may be carried out in fluidized bedpolymerizations (that may be mechanically stirred and/or gas fluidized),or with those utilizing a gas phase, similar to that as described above.In addition to well-known conventional gas phase polymerizationprocesses, it is within the scope of the present invention that“condensing mode”, including the “induced condensing mode” and “liquidmonomer” operation of a gas phase polymerization may be used.

Embodiments of the present invention may employ a condensing modepolymerization, such as those disclosed in U.S. Pat. Nos. 4,543,399;4,588,790; 4,994,534; 5,352,749; 5,462,999; and 6,489,408. Condensingmode processes may be used to achieve higher cooling capacities and,hence, higher reactor productivity. In addition to condensable fluids ofthe polymerization process itself, other condensable fluids inert to thepolymerization may be introduced to induce a condensing mode operation,such as by the processes described in U.S. Pat. No. 5,436,304.

Other embodiments of the present invention may also use a liquid monomerpolymerization mode such as those disclosed in U.S. Pat. No. 5,453,471;U.S. Ser. No. 08/510,375; PCT 95/09826 (US) and PCT 95/09827 (US). Whenoperating in the liquid monomer mode, liquid can be present throughoutthe entire polymer bed provided that the liquid monomer present in thebed is adsorbed on or in solid particulate matter present in the bed,such as polymer being produced or inert particulate material (e.g.,carbon black, silica, clay, talc, and mixtures thereof), so long asthere is no substantial amount of free liquid monomer present. Operatingin a liquid monomer mode may also make it possible to produce polymersin a gas phase reactor using monomers having condensation temperaturesmuch higher than the temperatures at which conventional polyolefins areproduced.

In one embodiment, a useful polymerization technique may be particleform polymerization or a slurry process where the temperature is keptbelow the temperature at which the polymer goes into solution. Otherslurry processes include those employing a loop reactor and thoseutilizing a plurality of stirred reactors in series, parallel, orcombinations thereof. Non-limiting examples of slurry processes includecontinuous loop or stirred tank processes. Also, other examples ofslurry processes are described in U.S. Pat. No. 4,613,484 and 2METALLOCENE-BASED POLYOLEFINS 322-332 (2000).

In one embodiment, a slurry polymerization process generally usespressures in the range of from 1 to 50 bar and even greater, andtemperatures in the range of 0° C. to 120° C. In a slurrypolymerization, a suspension of solid, particulate polymer is formed ina liquid polymerization diluent medium to which ethylene and comonomersand often hydrogen along with catalyst are added. The suspension,including diluent, is intermittently or continuously removed from thereactor where the volatile components are separated from the polymer andrecycled, optionally after a distillation, to the reactor. The liquiddiluent employed in the polymerization medium is typically an alkanehaving from 3 to 7 carbon atoms; in one embodiment, the alkane may be abranched alkane. The medium employed should be liquid under theconditions of polymerization and relatively inert. When a propane mediumis used, the process must be operated above the reaction diluentcritical temperature and pressure. In one embodiment, a hexane or anisobutane medium is employed.

In one embodiment of the process of the invention, the slurry or gasphase process may be operated in the presence of a metallocene-typecatalyst system and in the absence of, or essentially free of, anyscavengers, such as triethylaluminum, trimethylaluminum,tri-isobutylaluminum and tri-n-hexylaluminum and diethyl aluminumchloride, dibutyl zinc, and the like. By “essentially free” it is meantthat these compounds are not deliberately added to the reactor or anyreactor components, and if present, are present in the reactor at lessthan 1 ppm.

As noted above, the polymerization process of the present invention maybe carried out by using a solution process. Non-limiting examples ofsolution processes are described in, for example, U.S. Pat. Nos.4,271,060, 5,001,205, 5,236,998, and 5,589,555.

In another embodiment, one or all of the catalysts are combined with upto 15 weight percent of a metal-fatty acid compound, such as, forexample, an aluminum stearate, based upon the weight of the catalystsystem (or its components), such as disclosed in, for example, U.S. Pat.Nos. 6,300,436 and 5,283,278. Other suitable metals include other Group2 and Group 5-13 metals. In another embodiment, a solution of themetal-fatty acid compound is fed into the reactor. In anotherembodiment, the metal-fatty acid compound is mixed with the catalyst andfed into the reactor separately. These agents may be mixed with thecatalyst or may be fed into the reactor in a solution or slurry with orwithout the catalyst system or its components.

In some embodiments, for a fluidized bed gas-phase reactor, the reactortemperature of the fluidized bed process may be the highest temperaturethat is feasible taking into account the sticking temperature of thepolyolefin product within the reactor and any fouling that may occur inthe reactor or recycle line(s).

Polymer

In a class of embodiments, the polymers disclosed herein may have a meltindex (MI) or (I₂) as measured by ASTM-D-1238-E (190° C., 2.16 kgweight) in the range from 0.01 dg/min to 1000 dg/min. In otherembodiments, the polymer may have a MI from about 0.01 dg/min to about200 dg/min; from about 0.1 dg/min to about 200 dg/min in otherembodiments; and from about 1 dg/min to about 200 dg/min in yet otherembodiments.

In any of the embodiments described herein, the polymers disclosedherein may have a melt index ratio (MFR) (I₂₁/I₂, where I₂₁ is measuredby ASTM-D-1238-F, at 190° C., 21.6 kg weight) of from 5 to 300; fromabout 10 to less than 100 in other embodiments; from 15 to 50 in yetother embodiments; and from 15 to 40 in yet another embodiments.

In any of the embodiments described herein, the polymers describedherein may typically have a weight average molecular weight to numberaverage molecular weight (M_(w)/M_(n)) of greater than 1.5 to about 5,particularly greater than 2 to about 4.0, more preferably greater thanabout 2.2 to less than 3.5.

Hexane Extractables

In a class of embodiments, the hexane extractables content may be lessthan 1.75 percent; less than 1.5 percent in other embodiments; less than1.0 percent in yet other embodiments, and less than 0.5 percent in yetother embodiments. The data reported are measured in accordance withASTM D-5227

DSC Melting Point

In any of the embodiments described herein, DSC measurements may be madeon a Perkin Elmer System 7 Thermal Analysis System. The data reportedare from second melting data measured in accordance with ASTM 3418.

ESCR

ESCR is measured according to ASTM D-1693 Condition A (ASTM D-1693/A)and ASTM D-1693 Condition B (ASTM D-1693/B). For each condition,measurements are conducted in 10% and 100% Igepal. In embodiments, thepolymers described herein have an ESCR value of 100 hr or greater whenmeasured according to ASTM 1693/B in 10% Igepal. In other embodiments,the polymers described herein have an ESCR value of 250 hr or greaterand in yet other embodiments, the polymers described herein have an ESCRvalue of 500 hr or greater, when measured according to ASTM D-1693/B in10% Igepal.

In a class of embodiments, it has been found that resins produced withthe metallocene catalysts described herein that have a broadenedcomposition distribution characterized by having higher T₇₅-T₂₅ valueand a higher F₈₀ fraction possess substantially improved ESCR thancomparable grades with narrower composition distribution. Thepolyethylene grades described herein may have ESCR of greater than 500hr when measured according to ASTM D-1693/B measured in 100% Igepal.More details about the embodiments will be apparent from the examplesbelow.

Density

In any of the embodiments described herein, the density of the polymersmay be 0.927 g/cc or greater. In other embodiments, the density isbetween 0.927 g/cc and 0.965 g/cc and between 0.935 g/cc and 0.965 g/ccin yet other embodiments. Density is measured in accordance with ASTM D1505-03.

Melt Index

I₂₁ is measured in accordance with ASTM-D-1238-F (190° C., 21.6 kgweight).

I₅ is measured in accordance with ASTM-D-1238-G (190° C., 5 kg weight).

I₂ as measured in accordance with ASTM-D-1238-E (190° C., 2.16 kgweight).

The polyolefins of the present invention may be blended with otherpolymers and/or additives to form compositions that can then be used inarticles of manufacture. Appropriate additives, methods of adding themand methods of blending are known to the skilled artisan.

The polymers produced may further contain additives such as slip,antiblock, antioxidants, pigments, fillers, antifog, UV stabilizers,antistats, polymer processing aids, neutralizers, lubricants,surfactants, pigments, dyes and nucleating agents. Preferred additivesinclude silicon dioxide, synthetic silica, titanium dioxide,polydimethylsiloxane, calcium carbonate, metal stearates, calciumstearate, zinc stearate, talc, BaSO₄, diatomaceous earth, wax, carbonblack, flame retarding additives, low molecular weight resins,hydrocarbon resins, glass beads and the like. The additives may bepresent in the typically effective amounts well known in the art, suchas 0.001 weight % to 10 weight %.

The resultant polyolefin and polyolefin compositions may be furtherprocessed by any suitable means such as by calendering, casting,coating, compounding, extrusion, foaming; all forms of molding includingcompression molding, injection molding, blow molding, rotational molding(rotomolding), and transfer molding; film blowing or casting and allmethods of film formation to achieve, for example, uniaxial or biaxialorientation; thermoforming, as well as by lamination, pultrusion,protrusion, draw reduction, spinbonding, melt spinning, melt blowing,and other forms of fiber and nonwoven fabric formation, and combinationsthereof. Typical rotomolded articles include large containers forconveying liquids, drums, agricultural tanks, and large parts such ascanoes or large playground toys. Typical injection molded articlesinclude, housewares, thin wall containers, and lids for containers.

These and other forms of suitable processing techniques are describedin, for example, PLASTICS PROCESSING (Radian Corporation, Noyes DataCorp. 1986).

EXAMPLES

It is to be understood that while the invention has been described inconjunction with the specific embodiments thereof, the foregoingdescription is intended to illustrate and not limit the scope of theinvention. Other aspects, advantages and modifications will be apparentto those skilled in the art to which the invention pertains.

Therefore, the following examples are put forth so as to provide thoseskilled in the art with a complete disclosure and description of how tomake and use the compounds of the invention, and are not intended tolimit the scope of that which the inventors regard as their invention.

Example 1

Higher ESCR due to broadened orthogonal composition distribution

Polymerization

The ethylene/1-hexene copolymers were produced in accordance with thefollowing general procedure. The catalyst composition comprised a thesilica supported bis(n-propylcyclopentadienyl)hafnium dimethylmetallocene catalyst with methalumoxane, the Al:Hf ratio being fromabout 80:1 to 130:1, commercially available from Univation Technologies,LLC, Houston, Tex. The catalyst composition was injected dry into afluidized bed gas phase polymerization reactor. More particularly,polymerization was conducted in a 2590 mm diameter gas-phase fluidizedbed reactor operating at approximately 1720 kPa total pressure. Thereactor bed weight was approximately 17,000 kg. Fluidizing gascomprising ethylene, hydrogen, 1-hexene and nitrogen was passed throughthe bed at a velocity of approximately 0.6 m per second. The fluidizinggas exiting the bed entered a resin disengaging zone located at theupper portion of the reactor. The fluidizing gas then entered a recycleloop and passed through a cycle gas compressor and water-cooled heatexchanger. The shell side water temperature was adjusted to maintain thereaction temperature to the specified value. Ethylene, hydrogen,1-hexene and nitrogen were fed to the cycle gas loop just upstream ofthe compressor at quantities sufficient to maintain the desired gasconcentrations. Table 1 summarizes the gas concentrations and reactorconditions during the polymerizations.

Gas concentrations were measured by an on-line vapor fraction analyzer.Product (polyethylene particles) was withdrawn from the reactor in batchmode into a purging vessel before it was transferred into a product bin.Residual catalyst and activator in the resin was deactivated in theproduct drum with a wet nitrogen purge. Table 1 summarizes the gasconcentrations and reactor conditions during the polymerizations. “C₆/C₂flow ratio (“FR”)” is the ratio of the lbs of 1-hexene comonomer feed tothe pounds of ethylene feed to the reactor, whereas the C₆/C₂ ratio isthe ratio of the gas concentration of 1-hexene moles in the cycle gas tothe gas concentration of ethylene moles in the cycle gas. The C₆/C₂ratio is obtained from a cycle gas vapor fraction analyzer, whereas theC₆/C₂ Flow Ratio comes from some measure of the mass flow. The cycle gasis the gas in the reactor, and is measured from a tap off therecirculating loop around the reactor. The ratios reported in Table 1are from the gas concentrations in the reactor. The C₆/C₂ ratios arerunning averages. The STY reported in Table 1 is the Space Time Yield,the SGV is the Superficial Gas Velocity, and the APS is the AverageParticle Size of the resulting polymer. Table 2 summarizes the resultingpolyethylene properties. The comparative resin is a commercial resin(SURPASS™), available from Nova Chemicals. TABLE 1 Gas phasepolymerization of ethylene and 1-hexene with bis-(n-propylcyclopentadienyl) hafnium dimethyl catalyst. Parameter Sample 1Sample 2 Temp, ° C. 77 77 C₂ partial pressure, 234 219 psia Reactorpressure, psig 250 256 SGV, fps 2.2 2.20 Isopentane, mol % 5.0 10.6 C₆mol % 0.58 0.738 C₆/C₂ 0.0065 0.0091 C₆/C₂ FR (lb/lb) 0.022 0.0369 H₂ppm 309 370 H₂ ppm/C₂ mol % 3.51 4.58 Dew Point, ° C. 29.6 30.7 STY 5.46.82 Productivity, Cat 7283 13318 Feeder, g/g I₂, dg/min 6.03 6.50I₂₁/I₂ 17.26 21.1 Density, g/cm³ 0.9404 0.9383 APS, mm 0.0285 0.03

TABLE 2 Properties of Samples 1, 2 and the comparative sample 6Comparative Sample Parameter Sample 1 Sample 2 (Nova Surpass) I₂(dg/min) 6.03 6.50 5.3 I₂₁ (dg/min) 104.16 137.3 121.16 MFR (I₂₁/I₂)17.26 21.1 23.3 Density (g/cc) 0.9404 0.9383 0.939 Mn 32,673 29,49824,207 Mw 70,731 71,989 75,772 Mz 131,887 165,927 189,993 Mw/Mn 2.162.44 3.13 DSC Peak Melt 127.3 125.1 124.9 Temperature (° C.) T₇₅-T₂₅ (°C.) 2.45 5.3 4.20 F₈₀ 7.4% 11.75% 0% M₆₀/M₉₀ 4.6 2.89 N/A HexaneExtractables — 0.2 Conventional CD not measured ESCR (Condition B, 112hr >1000 hr  82 hr 10% Igepal) ESCR (Condition B, 247 hr >1000 hr 575 hr100% Igepal) ESCR (Condition A,  12 hr   35 hr  11 hr 10% Igepal) ESCR(Condition A,  94 hr >1000 hr 157 hr 100% Igepal)

Both Samples 1 and 2 were produced with embodiments of the metallocenecatalyst described herein and have a orthogonal composition distributionas evidenced by the M₆₀/M₉₀ value of greater than 2. Sample. 2 wasproduced with a broader composition distribution as evidenced by thehigher T₇₅-T₂₅ value, a higher low temperature fraction as evidenced bythe higher F₈₀ value both resulting in significantly improved ESCR. Thereactor conditions for both resins are given in Table 1. T₇₅-T₂₅ values,F₈₀ values, and ESCR are given in Table 2 along with other properties.TREF-LS data for both samples are given in FIGS. 1 and 2, respectively.

A comparative sample (SURPASS™ available from Nova Chemicals) of similarmelt index, density and T₇₅-T₂₅ value to Sample 2 is also shown in Table2. The comparative sample has a conventional composition distributionand no low temperature fraction. TREF-LS data for the comparative sampleis shown in FIG. 6.

While not whishing to be bound by theory, the inventors offer thefollowing explanation for the higher ESCR of Sample 2. It has long beenrecognized in the art that the presence of high molecular weight chainsthat contain the majority of the comonomer provide for increasedtoughness properties, especially ESCR. The high F₈₀ value in Sample 2 isevidence for such a fraction with higher comonomer content. Thesemolecules also possess a higher molecular weight than the molecules thatelute above 80° C. (i.e. molecules having lower comonomer content) asevidenced by an M₆₀/M₉₀ value of greater than 2 shown in the TREF LSdata in FIG. 2.

Both the orthogonal nature of the composition distribution and largerfraction of high molecular weight chains with increased comonomercontent provide for the improved ESCR of Sample 2 over the comparativesample. The larger fraction of high molecular weight chains withincreased comonomer content as evidenced by the higher F₈₀ value providefor improved ESCR of Sample 2 over Sample 1.

If the composition distribution was broad but not orthogonal, ESCR maybe disadvantageously low. Likewise, if the composition distribution wasorthogonal but too narrow, ESCR may be disadvantageously low as well.

Example 2

Resins having broad composition distributions

Samples 3, 4 and 5 were produced using the same reactor as described inExample 1. The reactor parameters and gas concentrations are listed inTable 3. The polymer properties are listed in Table 4. TABLE 3 Thereactor parameters and gas concentrations for production of samples 3, 4and 5. Parameter Sample 3 Sample 4 Sample 5 Temp, ° C. 85 85 85 C₂partial pressure, 235 148 149 psia Reactor pressure, psig 251 242 241SGV, fps 2.1 2.15 2.09 Isopentane, mol % 5.0 5.0 5.0 C₆ mol % 0.89 0.5230.687 C₆/C₂ 0.0101 0.0091 0.0118 C₆/C₂ FR (lb/lb) 0.072 0.0595 0.1059 H₂ppm 1416 849 1481 H₂ ppm/C₂ mol % 15.9 14.75 25.39 Dew Point, ° C. 33.525.9 26.9 STY 5.3 3.8 4.99 Productivity, Cat 13,048 7303 9753 Feeder,g/g I₂, dg/min 57.8 32.67 88.77 I₂₁/I₂ 23.9 23.73 21.4 Density, g/cm³0.932 0.9303 0.9297 APS, mm 0.027 0.03 0.02

TABLE 4 Properties of Samples 3, 4 and 5 Parameter Sample 3 Sample 4Sample 5 I₂ (dg/min) 56.7 23.5 116 I₂₁ (dg/min) 1289 643.9 — MFR(I₂₁/I₂) 22.7 27.4 — Density (g/cc) 0.9318 0.9288 0.9293 Mn 38627 16,05911,916 Mw 12304 50,307 33,817 Mz 105,090 64,062 Mw/Mn 3.14 3.13 2.84 DSCPeak Melt 122.0 122.9 122.2 Temperature (° C.) T₇₅-T₂₅ (° C.) 16.1 22.217.3 M₆₀/M₉₀ 1.33 2.00 1.1 Hexane Ext — 0.8 1.5

Samples 3, 4 and 5 show a broad composition evidenced by T₇₅-T₂₅ valuesof greater than 15. Moreover, Sample 4 shows an orthogonal compositiondistribution evidenced by a M₆₀/M₉₀ values of 2. The Hexane Extractablescontent of all samples is advantageously low.

Resins described herein having a broad orthogonal compositiondistribution show an advantageously improved ESCR over comparablecommercially available resins, as well as over resins having narrowercomposition distributions. Advantageously, the present inventionprovides for a method for the production of a polyethylene having abroad orthogonal composition distribution at densities of 0.927 g/cc orgreater.

The phrases, unless otherwise specified, “consists essentially of” and“consisting essentially of” do not exclude the presence of other steps,elements, or materials, whether or not, specifically mentioned in thisspecification, as along as such steps, elements, or materials, do notaffect the basic and novel characteristics of the invention,additionally, they do not exclude impurities normally associated withthe elements and materials used.

For the sake of brevity, only certain ranges are explicitly disclosedherein. However, ranges from any lower limit may be combined with anyupper limit to recite a range not explicitly recited, as well as, rangesfrom any lower limit may be combined with any other lower limit torecite a range not explicitly recited, in the same way, ranges from anyupper limit may be combined with any other upper limit to recite a rangenot explicitly recited. Additionally, within a range includes everypoint or individual value between its end points even though notexplicitly recited. Thus, every point or individual value may serve asits own lower or upper limit combined with any other point or individualvalue or any other lower or upper limit, to recite a range notexplicitly recited.

All priority documents are herein fully incorporated by reference forall jurisdictions in which such incorporation is permitted and to theextent such disclosure is consistent with the description of the presentinvention. Further, all documents and references cited herein, includingtesting procedures, publications, patents, journal articles, etc. areherein fully incorporated by reference for all jurisdictions in whichsuch incorporation is permitted and to the extent such disclosure isconsistent with the description of the present invention.

While the invention has been described with respect to a number ofembodiments and examples, those skilled in the art, having benefit ofthis disclosure, will appreciate that other embodiments can be devisedwhich do not depart from the scope and spirit of the invention asdisclosed herein.

1. A process to produce an ethylene alpha-olefin copolymer, the processcomprising: contacting ethylene and at least one alpha-olefin with ametallocene catalyst in at least one gas phase reactor at a reactorpressure of from 0.7 to 70 bar and a reactor temperature of from 20° C.to 150° C. to polymerize an ethylene alpha-olefin copolymer, wherein theethylene alpha-olefin copolymer satisfies the following conditions: adensity of 0.927 g/cc or greater, a melt index ratio of (I₂₁/I₂) of from15 to 40, an ESCR value of 500 hr or greater when measured according toASTM 1693/B in 10% Igepal, a T₇₅-T₂₅ value of 4 or greater wherein T₂₅is the temperature at which 25% of the eluted polymer is obtained andT₇₅ is the temperature at which 75% of the eluted polymer is obtained ina TREF experiment, and a F₈₀ value of 10% or greater, wherein F₈₀ is thefraction of polymer that elutes below 80 ° C.
 2. The process of claim 1,wherein the ethylene alpha-olefin copolymer has an ESCR value of 1000 hror greater when measured according to ASTM 1693/B in 10% Igepal.
 3. Theprocess of claim 1, wherein said density ranges from 0.927 g/cc to 0.965g/cc.
 4. The process of claim 1, wherein said density ranges from 0.935g/cc to 0.965 g/cc.
 5. The process of claim 1, wherein said densityranges from 0.940 g/cc to 0.965 g/cc.
 6. The process of claim 1, whereinsaid melt index ratio (I₂₁/I₂) ranges from 15 to
 25. 7. The process ofclaim 1, wherein the metallocene catalyst is selected from the groupconsisting of: bis(n-propylcyclopentadienyl)hafnium X_(n),bis(n-butylcyclopentadienyl)hafnium X_(n),bis(n-pentylcyclopentadienyl)hafnium X_(n), (n-propylcyclopentadienyl)(n-butylcyclopentadienyl)hafnium X_(n),bis[(2-trimethylsilylethyl)cyclopentadienyl]hafnium X_(n),bis(trimethylsilyl cyclopentadienyl)hafnium X_(n),dimethylsilylbis(n-propylcyclopentadienyl)hafnium X_(n),dimethylsilylbis(n-butylcyclopentadienyl)hafnium X_(n),bis(1-n-propyl-2-methylcyclopentadienyl)hafnium X_(n),(n-propylcyclopentadienyl)(1-n-propyl-3-n-butylcyclopentadienyl)hafniumX_(n); wherein X_(n) is selected from the group consisting of halogenions, hydrides, C₁₋₁₂ alkyls, C₂₋₁₂ alkenyls, C₆₋₁₂ aryls, C₇₋₂₀alkylaryls, C₁₋₁₂ alkoxys, C₆₋₁₆ aryloxys, C₇₋₁₈ alkylaryloxys, C₁₋₁₂fluoroalkyls, C₆₋₁₂ fluoroaryls, and C₁₋₁₂ heteroatom-containinghydrocarbons and substituted derivatives thereof.
 8. The process ofclaim 7, wherein the metallocene catalyst is a supported metallocenecatalyst.
 9. The process of claim 8, wherein the metallocene catalyst isactivated with at least one activator selected from the group consistingof an alumoxane, a modified alumoxane, an ionizing compound, or mixturesthereof.
 10. The process of claim 9, wherein the activator is analumoxane.
 11. An ethylene alpha-olefin copolymer, wherein the ethylenealpha-olefin copolymer satisfies the following conditions: a density of0.927 g/cc or greater, a melt index ratio of (I₂₁/I₂) of from 15 to 40dg/min, an ESCR value of 500 hr or greater when measured according toASTM 1693/B in 10% Igepal, a T₇₅-T₂₅ value of 4 or greater, and a F₈₀value of 10% or greater, wherein F₈₀ is the fraction of polymer thatelutes below 80° C.
 12. The ethylene alpha-olefin copolymer of claim 11,wherein the ESCR value is 1000 hr or greater when measured according toASTM 1693/B in 10% Igepal.
 13. The alpha-olefin copolymer of claim 11,wherein the density ranges from 0.927 g/cc to 0.965 g/cc.
 14. Thealpha-olefin copolymer of claim 11, wherein the density ranges from0.935 g/cc to 0.965 g/cc.
 15. The alpha-olefin copolymer of claim 11,wherein the density ranges from 0.940 g/cc to 0.965 g/cc.
 16. Thealpha-olefin copolymer of claim 11, wherein the melt index ratio(I₂₁/I₂) is from 15 to
 25. 17. A composition comprising the alpha-olefincopolymer of claim
 11. 18. An article obtained by rotational moldingcomprising the composition of claim 17, wherein the article has adensity of from 0.935 g/cc to 0.965 g/cc.
 19. The article of claim 18,wherein the article has a density of from 0.940 g/cc to 0.965 g/cc.